Illumination control system



July 16, 1935. F. R. ELDER ET AL ILLUMINATION CONTROL SYSTEM Filed July 9, 1932 Fig. 1.

CQRRENT MILLIAMPERES Inventors Fr'ank R Eicic-m Harold B. LaRoque,

Their- Attor'nc eg.

Patented July 16, 1935 ILLUMINATION CONTROL SYSTEM Frank R. Elder, Schenectady, and Harold B. La Roque, Scotla, N. Y., assignorl to General Electric Company, a corporation of New York Application July 9, 1932, Serial No. 621,820

12 Claims. (Cl. 178-124) This invention relates to illumination control systems, more particularly to systems for controlling the intensity of illumination of luminous discharge devices, such for example as commercial neon tubes, and it has for an object the provision of a simple, reliable, inexpensive and efllcient system of this character.

Luminous discharge devices of this type require a high breakdown voltage in order to initiate the discharge and for this purpose they are usually supplied with an alternating voltage through a step-up transformer. Due to the high break-down voltage required and to the interruption of the discharge each time the alternating voltage passes through zero, the problem of dimming tubes of this character is especially difficult. The dimming range with the known methods of resistance dimming is limited to a very small portion of the visible range of the tube, beyond which either a perceptible and intolerable flicker occurs or the tube is entirely darkened. Accordingly a further object of this invention is the provision of means for extending the dimming range of the tube, and more specifically to dim the tubes smoothly and gradually through substantially the full range of visibility from maximum brilliance to complete darkness without perceptible flicker.

In carrying the invention into effect in one form thereof a luminous discharge tube is supplied with an alternating voltage, and-means are provided for controlling and varying the point in the cycle of said voltage at which the discharge is initiated throughout substantially the entire portion of each half cycle of the applied voltage, thereby varying the intensity of illumination of the tube, gradually and smoothly from maximum brilliance to complete darkness. In a specific form of the invention, means preferably in the form of an impedance device are provided for preventing the tube voltage from attaining the break-down value during a predeterminedportion of the cycle of the alternating voltage, and additional means, preferably suitable electric valve apparatus, are provided for rendering the device ineflective and initiating the discharge at desired points in the tube voltage cycle and thereby controlling the intensity of illumination of the tube as desired.

For a better and more complete understanding of the invention, reference should now be had to the following specification and to the accompanying drawing in which Fig. l is a simple diagrammatical representation of an embodiment of the invention and Fig. 2 is a chart of curves illustrating certain tube operating characteristics and facilitating an understanding of the invention.

Referring now to the drawing, a cold cathode type gas-filled luminous discharge device "I, such for example as the well-known neon tube, is supplied with alternating voltage from a suitable source such for example as that represented by the three supply lines II by means of a suitable step-up transformer l2 to the terminals of the high initial voltage is necessary to ionize the included gas and overcome the drop at the electrode in each end of the tube. This ionization begins at the electrodes and practically instantaneously extends throughout the entire length of the tube, caFsing the gas to glow with its characteristic co or.

On account of the negative resistance characteristics of luminous discharge tubes, current flowing in the tube must be limited in order to prevent damage of the tube.

The relationship between the voltage existing at the terminals of the tube and the discharge current flowing in the tube is clearly shown by curve I! in Fig. 2 in which positiveordinates represent tube voltage and abscissa represent discharge current. A very high initial voltage represented by the peak I1. is required to break down the tube and initiate the-discharge. Thereafter, the voltage drops rapidly to a point ill, at which the current is suii'icient to render the discharge visible. From this point the current increases to a point 11c and it will be observed that this increase in current is accompanied by a decrease in voltage across the tube which is referred to as the negative resistance characteristic of the tube. The range between the points I11; and '11: is the useful glow range of the tube. If the current exceeds the value represented by the point 11c damage results to the tube. The current therefore, must be limited to a value between the points "a and '10 and this current limitation is obtained by designing the transformer l2 to. have a high inherent reactance and a relatively high exciting current. The high reactance current limiting characteristics of the transformer I2 is illustrated by curve It in Fig. 2 of the drawing in which it will be observed that the terminal voltage of the transformer decreases rapidly with increasing current to a point He at which the curve l8 crosses the curve ll. Thus, the discharge current of the tube is stabilized at a value represented by the point Ila. Furthermore, due to this design of the transformer, the voltage applied to the tube is steeply peaked so that the instantaneous values thereof are equal to or greater than the starting voltage of the tube through substantially the entire portion of each half cycle of the applied voltage. By refen'ing to the curve I! it will be clear that dimming the tube by means of resistance is limited to a very narrow range because as the resistance is increased the voltage applied to the tube is decreased until finally at no time in the cycle does it attain the value indicated at I'll necessary to break down the tube and initiate the discharge. That is to say, increasing the resistance in circuit with the tube decreases the instantaneous values of the voltage until finally even the maximum .peak value of they voltage is less than the starting voltage and consequently the discharge never starts at any time in the cycle with the result that the tube remains dark and the dimming range is limited to a very narrow range of high illumination intensities with no control at low intensities.

In order to extend the dimming range of the tube means illustrated as a transformer l9 having high impedance are provided for preventing the tube voltage from attaining the break-down value for a predetermined or variable portion of the alternating voltage cycle and additional means illustrated as suitable electric valve apparatus 20 are provided for rendering the impedance device I! inefiective at desired predetermined points in the tube voltage cycle. Control means 2| are provided for suitably controlling the operation of the electric valve apparatus 20.

As shown in the drawing, the primary winding of the transformer i9 is included in the connections l5, Hi to the supply source II in series'relationship with the primary winding of the tube transformer l2. The transformer I9 is designed to have an exciting current that is low with respect to the exciting current required by the transformer l2 to induce the tube break-down voltage in the secondary winding thereof and is also designed to have an open circuit impedance that is large with respect to the impedance of the transformer l2.

Although the electric valve apparatus 20 may be of any suitable type'it is illustrated as a pair of electric discharge devices 22 and 23 each preferably of the three-electrode vapor electric discharge type connected for full wave rectification. As shown the device 22 is provided with an anode 24, a control grid 25 and a cathode 26 whilst the device 23 is similarly provided with an anode 21, a control grid 28 and a cathode 29. The cathodes 26 and 29 are connected together as shown and the anodes 24 and 21 are respectively connected to opposite terminals of the secondary winding of the transformer is, the mid-point of which is connected to the mid-point between the cathodes 28 and 28 by means of the conductor 30. During one half cycle of the alternating voltage of the secondary winding of the transformer is, current flows from the anode 21 to the cathode 29, thence through conductor 30 and the right-hand half of the secondary winding of the transformer I3 whilst in the opposite half cycle current flows from the anode 23 to the cathode 25 through conductor 30 and thence through the left-hand half of the secondary winding of the transformer It. It will thus be seen that the electric valve apparatus rectifles both half cycles of the alternating voltage wave.

For the purpose of controlling the initiation of current flow in the anode circuit of the electric valve apparatus 20 during each positive half cycle of the voltages applied to the anodes 24 and 21 respectively a suitable control voltage is and 33.

Although the control device 2| may be of anysuitable character, it is illustrated as a device for shifting the phase of the grid voltage of the electric valve apparatus 23 with respect to the anode 25 voltage. As shown, it comprises a stator winding 34 and a rotor member provided with a winding 35 in inductive relation with the stator winding. The stator winding 34 is physically similar to the polyphase winding of a polyphase dynamo electric machine and its terminals are respectively connected to any suitable source such for example as the three supply lines ll, whilst the terminals of the rotor winding 35 are connected to the terminals of the primary winding of the grid transformer 3L.

The current flowing in the primary winding 34 produces a rotating magnetic field which induces a voltage in the secondary winding 35 and this voltage is applied to the grids 25 and 23 of the electric valve apparatus 20. Rotation of the rotor winding 35 in space changes the phase relationship of the voltage applied to the grid with respect to theanode voltage. When the grid voltage is exactly in phase with the anode voltage current begins to flow in the anode circuit as the anode voltage passes through zero and attains a positive value; whereas when the grid voltage is 180 out of phase with the anode voltage curent does not flow in the anode circuit at any time. For intermediate phaserelationships of the grid and anode voltages, the flow of current in the anode circuit is initiated at correspondingly proportionate points in the cycle of the anode voltage.

' With the above understanding of the apparatus and its organization in the completed system, the operation will readily and easily be understood from the detailed description which follows:

Assuming that the rotor 35 of the control 2| occupies such a position in space that the grid voltage of the electric valve apparatus 23 is 180 out of phase with the anode voltage, flow of current in the anode circuit will not be initiated at any time in the cycle of the anode voltage and consequently the transformer is behaves as a transformer with an open-circuited secondary winding and the reactance of its primary winding is so great that practically the entire line voltage is absorbed between its terminals with the result that substantially no voltage exists across the terminals of the primary winding of the tube transformer l2. Consequently, the voltage applied to the electrode l0. and Ila never attains the breakdown value l1. and as a result the tube l0 remains dark throughout the entire cycle oi the applied voltage.

Rotation oi the rotor winding 35 01 the phase shifting device 2| varies the phase relationship of the grid voltage of the electric valve apparatus 20 with respect to the anode voltage. Assuming that the rotor winding 35 is rotated in such a direction as to shift the phase of the grid voltage toward the inphase position with the anode voltage, current flow in the anode circuit of the electric valve apparatus is established at a point in the cycle or the anode voltage depending upon the amount oi. rotation of the winding 35. At the instant that current flow is established in the anode circuit, the transformer I9 no longer acts as a transformer with an opencircuited secondary but on the other hand behaves as a transformer with a short-circuited secondary with the result that impedance is greatly diminished and there is a proportionate decrease in the voltage drop across the primary winding. This decrease in the voltage drop across the primary winding has the effect of immediately increasing the voltage drop across the terminals of the primary winding and of the secondary winding of the tube transformer l2 to such an extent that the break-down voltage '19. is applied to the cathodes I0; and Hit of the tube I0, thereby initiating the discharge which is maintained during the remainder of the half cycle of the applied voltage until the latter passes through zero.

The above described operation is of course repeated during each half cycle of the applied voltage and consequently the brilliancy of the discharge of the tube In as it appears to the eye is the integrated brilliancy of the tube for the portions of the voltage cycle during which there is a discharge. That is to say, if there is no discharge at any time the tube is completely dark and if the discharge is initiated at or near the beginning of the half cycle the tube appears to have maximum brilliancy whereas for a discharge initiated at intermediate points in the cycle the tube appears to have a proportionate intermediate brilliancy.

It will thus be seen that by varying the position of the rotor winding 35 relative to the stator winding 34 the instant in the half cycle of the tube voltage wave at which the discharge is initiated is controlled as desired and the intensity of illumination of the luminous discharge tube ill likewise controlled as desired from maximum brilliancy to complete darkness without any perceptible flicker.

Although in accordance with the provision of the patent statutes, the invention is described as embodied in concrete form, the specific apparatus and connections shown are merely illustrative and the invention is not limited thereto since alterations and modifications will readily suggest themselves to persons skilled in the art without departing from the true spirit of the invention or the scope of the annexed claims.

What we claim as new and desire to secure by Letters Patent of the United States, is:

1. An illumination control system comprising a luminous discharge tube having electrodes for supporting a discharge, means for supplying an alternating voltage to said electrodes, and means for controlling the intensity of illumination of said tube comprising means external to said tube for controlling said voltage so as to vary the instant at which the discharge is initiated throughout substantially the entire portion of each half cycle of said alternating voltage and for sustaining said discharge substantially throughout remaining portions of each half cycle.

2. An illumination control system comprising a luminous discharge tube having predetermined starting and cut of! voltages, said tube having electrodes for supporting a discharge, means for applying an alternating voltage to said electrode having instantaneous values equal to or greater than said starting voltage over substantially the entire portion oi. each half of the voltage wave and means external to said tube for preventing the instantaneous values of applied voltage from attaining said predetermined starting voltage throughout substantially the entire portion of each half cycle of the voltage wave and thereafter applying instantaneous values of voltage greater than said cut oil voltage for substantially the remainder of each half of the voltage wave.

3. An illumination control system comprising a luminous discharge tube having predetermined starting and cut 01? voltages, said tube having electrodes for supporting a discharge, means for applying an alternating voltage to said electrodes having instantaneous values equal to or greater than said starting voltage over substantially the entire portion oi each half cycle of said alternating voltage, means said starting voltage, and means external to said tube for rendering said voltage reducing means ineffective at any selected instant during substantiallythe entire half cycle of said alternating voltage so that full instantaneous values of said voltage greater than said cut off value are applied to initiate and maintain the discharge through the remainder oi. each half cycle of the voltage.

4. An illumination control system comprising a luminous discharge tube having predetermined starting and cut oil voltages, said tube having electrodes for supporting a discharge, means for applying an alternating voltage to said electrodes having instantaneous values equal to or greater than said predetermined voltage over substantially the entire portion of each half cycle of the applied voltage, means for decreasing the instantaneous values of said applied voltage below said starting voltage, control means external to said tube for rendering said voltage reducing means inefiective at a selected instant during each half cycle of the applied voltage so that instantaneous values of voltage above said out off voltage are applied to initiate and sustain a discharge through said tube during the remainder of each half cycle, and means for controlling said control means to vary the instant at which said voltage reducing means is rendered ineffective through substantially the entire portion of each half cycle of the applied voltage.

5. An illumination control system comprising a cold cathode luminous discharge tube, means for supplying an alternating voltage to said tube, and means for gradually varying the intensity of illumination of said tube comprising electric valve apparatus for determining the instant dur-' ing half cycles of said voltage for initiating the discharge of said tube and for sustaining said discharge through substantially the remaining portions of said half cycles.

6. An illumination control system comprising a luminous discharge tube having electrodes for supporting a discharge, means for supplying alternating voltage to said electrodes, and means for decreasing the in- I stantaneous values of said applied voltage below for controlling the intensity of illumination of said tube comprising a device external to said tube for controlling the voltage supplied to said electrodes to prevent discharge in said tube and means for rendering said device inactive at any predetermined instant throughout substantially the entire cycle of said alternating voltage and for maintaining said device inactive throughout substantially the remaining portion of said cycle.

'7. An illumination control system comprising in' combination a cold cathode luminous discharge tube, means for supplying an alternating voltage to said tube, and means for controlling I the intensity of illumination oi. said tube comrising a reactance device for preventing the discharge of said tube, and means for rendering said reactance device ineffective at any predetermined point throughout substantially the entire cycle of said alternating voltage and for maintaining said reactance device ineffective through substantially the remaining portion of said cycle.

8. An illumination control system comprising in combination, a luminous discharge tube, means for supplying an alternating voltage to said tube, and means for controlling said tube comprising a reactance device for limiting the voltage of said tube to a value below that at which a discharge is initiated, electric valve apparatus for rendering said reactance device ineffective at predetermined points in the cycle of said alter nating voltage, and means for controlling the operation of said valve apparatus.

9. An illumination control system comprising a luminous discharge device, means for supplying alternating voltage to said device comprising connections to a supply source including a transformer, a second transformer included in said connections having a magnetizing current materially less than that of said first mentioned transformer and having a normally open-circuited secondary winding, and means for controlling said device comprising means for short-circuiting said secondary winding at predetermined points in the cycle of said alternating voltage.

10. In an illumination control system for a luminous discharge tube, means for supplying an alternating voltage to said tube comprising electrical connections including a transformer, a second transformer included in said connections and having an impedance sufficiently large to prevent discharge in said tube, and means for reducing the impedance of said second transformer to initiate the discharge in said tube at predetermined points in the cycle of said alternating voltage.

11. In an illumination control system for a luminous discharge tube, means for supplying an alternating voltage to said tube comprising connections to a supply source and a transformer included in said connections, a. second transformer included in said connections and having an open circuit impedance of sufllcient magnitudeto prevent the discharge of said tube, and means for controlling the intensity of illumination of said tube comprising electric valve apparatus connected to said second transformer and provided with a. control grid, and phase shifting means connected to said grid for controlling said electric valve apparatus to reduce the impedance of said second transformer at predetermined points in the cycle of said alternating voltage thereby to initiate the discharge in said tube.

12. In an illumination control system for a luminous discharge tube, means for supplying an alternating voltage to said tube comprising a transformer having primary power source connections and having its secondary winding connected to said tube, a second transformer having the primary winding included in said connections in series relationship with the primary'winding of said tube transformer and having its secondary winding normally open-circuited, electric valve apparatus-provided with an anode connected to said open circuited secondary winding and also provided with a control grid supplied with alternating voltage, and means for controlling the intensity of illumination of said tube comprising means for varying the phase of said grid voltage relative to the voltage of said anode thereby to provide initiation of the discharge in said tube at predetermined points in the cycle of said alternating voltage.

FRANK R. ELDER. HAROLD B. LA ROQUE. 

